A device conventionally known as a two-dimensional image detector for detecting an image by radiation has such a structure that semiconductor sensors for sensing X-rays to generate charges (electrons-holes) are two-dimensionally disposed, and electric switches are provided for the respective sensors so as to read out the charges of the sensors column by column by sequentially turning on the electric switches row by row. The specific structure and the principle of the two-dimensional image detector are described in, for example, references: D. L. Lee, et al., "A New Digital Detector for Projection Radiography", SPIE, 2432, pp. 237-249, 1995; and L. S. Jeromin, et al., "Application of a-Si Active-Matrix Technology in a X-Ray Detector Panel", SID 97 DIGEST, pp. 91-94, 1997; and Japanese Publication of Unexamined Patent Application No. 342098/1994 (Tokukaihei 6-342098).
The following descriptions will explain the structure and principle of the conventional two-dimensional radiation image detector mentioned above. FIG. 8 is a depiction of the structure of the two-dimensional radiation image detector. FIG. 9 is a depiction of the cross section showing the structure of each pixel of the two-dimensional radiation image detector.
As shown in FIGS. 8 and 9, the two-dimensional radiation image detector includes an active matrix substrate having a glass substrate 51 on which XY matrix-form electrode wiring (gate electrodes 52 and source electrodes 53), TFTs (thin film transistors) 54, and charge storage capacitors (Cs) 55, etc. are formed. On the almost entire surface of the active matrix substrate, a photoconductive film 56, a dielectric layer 57, and a top electrode 58 are formed.
The charge storage capacitor 55 includes a Cs electrode 59 and a pixel electrode 60 connected to a drain electrode of the TFT 54, arranged so that the Cs electrode 59 faces the pixel electrode 60 through an insulating film 61.
The photoconductive film 56 is formed by a semiconductor material for generating charges on exposure to radiation such as X-rays. According to the above references, amorphous selenium (a-Se) having a high dark resistance and showing satisfactory photoconduction characteristics on exposure to X-ray irradiation is used. The photoconductive film 56 is formed by a vacuum evaporation method to have a thickness ranging from 300 to 600 .mu.m.
As the above-mentioned active matrix substrate, an active matrix substrate formed in the process of manufacturing a liquid crystal display device can be utilized. For example, an active matrix substrate used for an active-matrix type liquid crystal display device (AMLCD) includes TFTs formed by amorphous silicon (a-Si) or poly-silicon (p-Si), XY matrix electrodes, and charge storage capacitors. Therefore, by only slightly changing the design, the active matrix substrate formed in the process of manufacturing the liquid crystal display device can be easily utilized as the active matrix substrate for use in the two-dimensional radiation image detector.
Next, the following descriptions will explain the operational principle of the two-dimensional radiation image detector having the above-mentioned structure.
When the photoconductive film 56 is exposed to radiation, charges are generated in the photoconductive film 56. As shown in FIGS. 8 and 9, since the photoconductive film 56 and the charge storage capacitor 55 are electrically connected in series, when a voltage is applied across the top electrode 58 and the Cs electrode 59, the negative and positive charges generated in the photoconductive film 56 move toward the anode side and the cathode side, respectively. As a result, the charges are accumulated in the charge storage capacitor 55. Note that a charge blocking layer 62 as a thin insulating layer is formed between the photoconductive film 56 and the charge storage capacitor 55, and functions as a blocking-type photodiode for blocking the flow of the charges from one side.
With this function, the charges accumulated in the charge storage capacitors 55 can be taken out via source electrodes S1, S2, S3, . . . , Sn by setting the TFTs 54 to the open state in accordance with input signals of gate electrodes G1, G2, G3, . . . , Gn. Since the gate electrodes 52, the source electrodes 53, the TFTs 54, the charge storage capacitors 55, etc. are all provided in the XY-matrix form, X-ray image information can be obtained two-dimensionally by sequentially scanning the signals inputted to the gate electrodes G1, G2, G3, . . . , Gn line by line.
If the photoconductive film 56 used in the above two-dimensional image detector shows photoconductivity with respect to visible light and infrared light as well as radiation such as X-rays, the two-dimensional image detector also functions as a two-dimensional image detector for detecting an image by visible light and infrared light.
However, in the above conventional structure, a-Se is used as the photoconductive film 56. The response of a-Se is not good because the photocurrent produced by a-Se has distributed conduction characteristics typical of amorphous materials. Further, since the sensitivity (S/N ratio) of a-Se to X-rays is not sufficient, information cannot be read out until the charge storage capacitors 55 are fully charged by long-time irradiation with X-rays.
In addition, the dielectric layer 57 is provided between the photoconductive film 56 and the top electrode 58 for reduction of the leakage current (dark current) and protection from the high voltage. Since the charges remain in the dielectric layer 57, a sequence for removing the remaining charges every frame must be added, thereby causing such a problem that the two-dimensional image detector can be utilized for only shooting static images.
Meanwhile, in order to obtain image data corresponding to dynamic images, it is necessary to use the photoconductive film 56 made of a crystalline (or polycrystalline) photoconductive material having excellent sensitivity (S/N ratio) to X-rays in place of a-Se. When the sensitivity of the photoconductive film 56 is improved, the charge storage capacitors 55 can be sufficiently charged even by short-time irradiation with X-rays, and the application of a high voltage to the photoconductive film 56 becomes unnecessary, thereby eliminating the need for providing the dielectric layer 57. As a result, adding the sequence for removing the remaining charges every frame is not required, thereby making it possible to shoot dynamic images.
Photoconductive materials known to have excellent sensitivity to X-rays are CdTe, CdZnTe, etc. In general, X-ray photoelectric absorption of a material is proportional to the fifth power of the effective atomic number of the absorbing material. For example, provided that the atomic number of Se is 34 and the effective atomic number of CdTe is 50, about 6.9-times improvement in sensitivity can be expected. However, when using CdTe or CdZnTe as the photoconductive film 56 of the two-dimensional radiation image detector instead of a-Se, the following problem arises.
In the conventional case of using a-Se, the vacuum evaporation method can be employed as the method for depositing the film, and the film can be deposited at room temperatures. Thus, the film deposition on the active matrix substrate was easy. Meanwhile, in the case of using CdTe and CdZnTe, the MBE method and the MOCVD method are known as methods for depositing the film, and particularly, the MOCVD method is suitable, considering deposition of the film over the large-area substrate.
However, the deposition of CdTe or CdZnTe by the MOCVD method requires a high temperature of about 400.degree. C., because thermal decomposition of organic cadmium (DMCd) as a raw material occurs at about 300.degree. C., and thermal decomposition of organic telluriums (DETe and DiPTe) as raw materials occurs at about 400.degree. C. and 350.degree. C., respectively.
Generally, the above-mentioned TFT 54 formed on the active matrix substrate includes an a-Si film or a p-Si film as a semiconductor layer. These films are deposited at temperatures ranging from about 300 to 350.degree. C. while adding hydrogen (H.sub.2), so as to improve their semiconductor characteristics. The TFT 54 formed in this way has a heat resistance up to about 300.degree. C. Accordingly, treating the TFT 54 at temperatures higher than 300.degree. C. causes hydrogen to come out of the a-Si film or the p-Si film, thereby deteriorating the semiconductor characteristics.
Consequently, the film deposition of CdTe or CdZnTe by the MOCVD method on the active matrix substrate was practically difficult in view of the film deposition temperature.